Mammographic density is one of the strongest risk factors for the development of breast cancer. It is thought in part to reflect the proliferative stte of epithelial and stromal cells in the breast and has been found to be associated with levels of some endogenous hormones and growth factors. It is reduced by tamoxifen therapy and by a gonadotropin-releasing hormone agonist and is increased with use of estrogen plus progesterone. Mammographic density, therefore, may be associated with both the risk and progression of breast cancer. A few, very small studies suggest mammographic density may predict recurrence among patients with invasive breast cancer. Our primary goal is to determine whether mammographic density at diagnosis of invasive breast cancer predicts risk of subsequent breast cancer events among patients participating in two large and well-characterized population-based cohorts (LACE and Pathways) within the Kaiser Permanente Northern California (KPNC) health plan membership. The combined LACE and Pathways cohorts are an ideal study population for this proposal because of the availability of patient (e.g, age, body mass index, race/ethnicity), clinical (e.g., disease stage, treatment) and behavioral factors (e.g., weight loss), as well as tumor marker data for classification of cancers into intrinic subtypes. At KPNC, members receive virtually all their health care from the plan. Further, all contents of the medical record, including mammograms, are stored indefinitely and are available for IRB- approved research. The specific aims are to: 1) Examine the association between mammographic density and breast cancer intrinsic subtypes; 2) Examine the association between mammographic density and risk of recurrence and new primary disease; 3) Examine whether the association between mammographic density and prognosis varies by intrinsic subtype; and 4) Examine if change in density after initiation of adjuvant tamoxifen predicts treatment benefit. This will be the largest study to date on mammographic density and prognosis in patients with invasive breast cancer. It also is the first to combine measures of mammographic density with molecular classification of tumors using a 50-gene assay, enabling us to examine whether associations of breast density and prognosis vary by intrinsic subtype.